The present invention relates to magnetic ceramics that have low loss and that can be used in higher frequencies (hereinafter referred to as xe2x80x9chigh-frequency magnetic ceramicxe2x80x9d). The present invention also relates to high-frequency circuit components including the magnetic ceramics. The high-frequency circuit components function as non-reciprocal circuit elements such as circulators and isolators.
For several years, as seen in the expanding mobile communication market, higher frequencies have been used in information and communication fields with trends toward higher communication rates and densities. Typical high-frequency circuit components (hereinafter referred to as xe2x80x9ccircuit componentsxe2x80x9d) used in these fields are non-reciprocal circuit elements such as circulators and isolators. Increasing requirements for these circuit components are lower loss, smaller sizes, broader bands, and lower prices.
Compact mobile communication devices such as portable phones require circuit components having low loss so that they can be driven by batteries for longer times. Yttrium-iron-based garnet materials having high electrical resistances and low loss at high frequencies are used in high-frequency magnetic ceramics for the mobile communication devices.
In general, the high-frequency magnetic ceramics must satisfy the following characteristics: a desired saturation magnetization Ms depending on the frequency to be used; a high curie temperature Tc, a small half width xcex94H of ferromagnetic resonance magnetic field, and a small dielectric loss. It is preferable that the half width xcex94H be smaller to achieve circuit components with low loss.
Unfortunately, it is difficult to appropriately control the saturation magnetization Ms depending on the frequency used without an increase in the half width xcex94H. In other words, no known high-frequency magnetic ceramics satisfy the above characteristics, and thus, high-frequency circuit components including the high-frequency magnetic ceramics do not show satisfactory characteristics. According to the investigation by the present inventors, the half width xcex94H in known high-frequency magnetic ceramics significantly increases depending on their compositions, relative densities, average crystal grain diameters, and 3"sgr" wherein "sgr" represents the standard deviation of the grain diameters, so that the magnetic ceramics cause often troubles in use.
Accordingly, an object of the present invention is to provide a high-frequency magnetic ceramic that has a small half width xcex94H and a desired saturation magnetization Ms.
Another object of the present invention is to provide a high-frequency circuit component having superior electrical characteristics such as low loss.
Disclosure of Invention
A high-frequency magnetic ceramic comprises garnet ferrite represented by the formula AZB8-ZO12 wherein the site A comprises yttrium and the site B comprises iron, wherein Z is in the range of more than 3.00 to 3.09, the relative density (the ratio of the density of the sintered ceramic to the theoretical density) of the high-frequency magnetic ceramic is at least 95%, the average grain diameter is at least 3 xcexcm, and the 3"sgr" value of the grain diameters is 2 xcexcm or less wherein "sgr" represents the standard deviation of the grain diameters. Preferably, the site A further comprises calcium and the site B further comprises indium and vanadium.
In case of Z less than 3.00, the half width xcex94H undesirably increases compared with the case of Z=3.00 which is the stoichiometric value in the formula.
Also in case of Z greater than 3.09, the half width xcex94H undesirably increases compared with the case of Z=3.00. Thus, Z is in the range of more than 3.00 to 3.09 in the present invention. At a relative density of less than 95%, the magnetic ceramic has many pores. Magnetic poles occurring around the pores generate a demagnetization field, which extends a resonance magnetic field, resulting in an increase in half width xcex94H. Thus, the relative density in the present invention is at least 95%.
At an average grain diameter of less than 3 xcexcm, each crystal grain has a large anisotropic magnetic field, resulting in an increase in half width xcex94H. Thus, the average grain diameter in the present invention is at least 3 xcexcm. In case of comparing several sintered ceramics having the same average grain diameter with each other, a 3"sgr" value exceeding 2 xcexcm causes noticeable magnetic field nonuniformity which extends the resonance magnetic field, resulting in an increase in half width xcex94H. Thus, the 3"sgr"value in the present invention is 2 xcexcm or less.
As described above, in the present invention, Z is in the range of more than 3.00 to 3.09, the relative density is at least 95%, the average grain diameter is at least 3 xcexcm, and the 3"sgr" value of the grain diameters is 2 xcexcm or less. In a magnetic ceramic satisfying these conditions, the saturation magnetization Ms can be controlled over a wide range, while maintaining a small half width xcex94H. Thus, a high-frequency circuit component having such a configuration exhibits low loss in higher frequencies.
In case that the site A in the formula further comprises calcium and the site B further comprises indium and vanadium, the molar ratio Ca/Y of calcium to yttrium in the site A is preferably in the range of 0.266 to 0.351. At a molar ratio outside of this range, foreign phases are generated in the primary phase represented by the above formula, resulting in the upward tend of the half width xcex94H.
Preferably, the site B in the formula comprises 4.517 to 4.607 moles of iron, 0.04 to 0.08 mole of indium, and 0.30 to 0.36 mole of vanadium. If any one of these elements is outside of the above range, foreign phases are generated in the primary phase represented by the above formula, resulting in the upward tend of the half width xcex94H. More preferably, the site B comprises 4.517 to 4.607 moles of iron, 0.060 mole of indium, and 0.333 mole of vanadium.
In case that the site A in the formula further comprises calcium and the site B further comprises vanadium, the molar ratio Ca/V of calcium to vanadium is preferably in the range of 2.00 to 2.40 and more preferably in the range of 2.01 to 2.40. At a molar ratio within this range, the green magnetic ceramic can be sintered at lower temperatures and the resulting sintered magnetic ceramic exhibits low loss.
A high-frequency circuit component according to the present invention comprises a main body comprising the above high-frequency magnetic ceramic; a plurality of central conductors that intersect each other and are electrically insulated from each other in the main body; and a magnetic field generator for applying a DC magnetic field to the main body and the central conductors.
In this configuration, the electric field of microwave power in the main body of the high-frequency circuit component can be controlled by adjusting the magnitude and the vector of the DC magnetic field; thus, microwave power input from a central conductor (first central conductor) can be output through another central conductor or the other central conductors.
Furthermore, microwave power input from another central conductor (second central conductor) can also be output through another central conductor or the other central conductors (including the first central conductor in this case).
Accordingly, the above configuration is applicable to non-reciprocal circuit elements such as circulators and isolators.